The present invention relates to a process for the production of vinyl acetate.
Vinyl acetate is generally prepared on a commercial basis by contacting acetic acid and ethylene with molecular oxygen in the presence of a catalyst active for the production of vinyl acetate.
A catalyst suitable for use in the production of vinyl acetate may comprise a Group VIII metal, a catalyst promoter and an optional co-promoter. For example EP A-0672453 discloses a process for the production of vinyl acetate by the catalytic oxidation of ethylene in the presence of acetic acid wherein the catalyst is a supported palladium catalyst comprising a promoter and a co-promoter.
It has been found that when the catalyst is used in a fluid bed reactor and the acetic acid is introduced into the reactor in the liquid form, the catalyst particles can become sticky and join together, thus forming lumps especially over extended periods of operation. In extreme cases, fluidisation can be lost which may cause the reactor to be shut down.
European patent publication EP-0847982-A relates to a fluid bed process for the production of vinyl acetate in which liquid is introduced into the fluidised bed reactor for the purpose of removing heat. The liquid introduced into the fluidised bed may be a reactant including the acetic acid reactant. Promoter such as potassium acetate may be dissolved in the liquid feed to the bed. In the example described therein the catalyst had a metal loading of 0.44 Pd, 0.36 Au and 2.5 K (weight %). This amount of potassium corresponds to 6.3 weight % potassium acetate. Although it is stated that the catalyst did not agglomerate and defluidise there is no indication of the scale of the experiment or how long the experiment was performed.
We have found that the problem of stickiness can be avoided by limiting the amount of co-promoter material in the catalyst composition.
Accordingly, the present invention provides a process for the production of vinyl acetate which comprises feeding ethylene, liquid acetic acid and an oxygen-containing gas into a fluid bed reactor and reacting at elevated temperature in the fluid bed reactor the ethylene, acetic acid and oxygen in the presence of a fluid bed catalyst material, said catalyst material comprising a Group VIII metal, a promoter and a co-promoter, said co-promoter being present in the reactor in an amount of up to 6% by weight of the catalyst.
The present invention provides a process for the production of vinyl acetate which avoids the problem of catalyst stickiness and ultimate lump formation, especially over extended periods of operation (for example greater than 7 days).
The catalyst of the present invention is a fluid bed catalyst material, comprising a Group VIII metal, a promoter and a co-promoter. These compounds are suitably accommodated on a support.
With regards to the Group VIII metal, the preferred metal is palladium. Suitable sources of palladium include palladium (II) chloride, sodium or potassium tetrachloropalladate, (II), (Na2PdCl4 or K2PdCI4), palladium acetate, H2PdCl4, palladium (II) nitrate or palladium (II) sulphate. The metal may be present in a concentration of greater than 0.2% by weight, preferably greater than 0.5% by weight, especially about 1% by weight based upon total weight of catalyst. The metal concentration may be as high as 10% by weight.
In addition to the Group VIII metal, the catalyst comprises a promoter. Suitable promoters include gold, copper, cadmium and/or nickel compounds. A preferred promoter is gold. Suitable sources of gold include gold chloride, tetrachloroauric acid (HAuCl4), NaAuCl4, KAuCl4, dimethyl gold acetate, barium acetoaurate or gold acetate. The preferred gold compound is HAuCl4. The promoter metal may be present in an amount of from 0.1 to 10% by weight in the finished catalyst.
The catalyst composition comprises a co-promoter material present in a concentration of up to 6% by weight of the catalyst composition. Suitable co-promoters include Group I, Group II, lanthanide or transition metals, for example cadmium, barium, potassium, sodium, iron, manganese, nickel, antimony, and/or lanthanum, which are present in the finished catalyst as salts, e.g. an acetate salt. The preferred salts are potassium or sodium acetate. The co-promoter is present in the catalyst composition in a concentration of up to 6% by weight of catalyst. Preferably, the concentration is from 3.5 to 5.5% by weight of catalyst, especially about 5% by weight.
The catalyst material is a supported catalyst. Suitable catalyst supports include porous silica, alumina, silica/alumina, titania, silica/titania, zirconia or carbon. Preferably the support is silica. Suitably, the support may have a pore volume from 0.2 to 3.5 mL per gram of support, a surface area of 5 to 800 m2 per gram of support and an apparent bulk density of 0.3 to 1.5 g/mL. The support may typically have a particle size distribution such that at least 60% of the catalyst particles have a particle diameter of below 2xc3x9710xe2x88x924m (200 microns). Preferably at least 50%, more preferably, at least 80%, most preferably, at least 90% of the catalyst particles have a particle diameter less than 1.05xc3x9710xe2x88x924m (105 microns). Preferably no more than 40% of the catalyst particles have a diameter of less than 4xc3x9710xe2x88x925m (40 microns).
The catalyst may be prepared by any suitable method, such as that detailed in EP-A-0672453. Suitably, the first stage of the catalyst preparation process involves impregnation of the support material with a solution containing the required Group VIII metal and the promoter metal in the form of soluble salts. Examples of such salts are soluble halide derivatives. The impregnating solution is preferably an aqueous solution and the volume of solution used is such that it corresponds to between 50 and 100% of the pore volume of the support, preferably 50 to 99% of the pore volume.
The impregnated support is dried at ambient or reduced pressure and from ambient temperature to 150xc2x0 C., preferably 60 to 130xc2x0 C. prior to metals reduction. To convert such materials into the metallic state, the impregnated support is treated with a reducing agent such as ethylene, hydrazine, formaldehyde or hydrogen. If hydrogen is used, it will usually be necessary to heat the catalyst to 100 to 850xc2x0 C. in order to effect complete reduction.
After the steps described above have been carried out, the reduced catalyst is washed with water and then dried. The dried carrier is then impregnated with the required amount of co-promoter and thereafter dried. Alternatively, the wet, reduced, washed material is impregnated with co-promoter then dried.
The method of catalyst preparation may be varied to optimise catalyst performance based on maximising vinyl acetate yield and selectivity.
The process of the present invention comprises reacting ethylene, acetic acid and an oxygen-containing gas in the presence of the catalyst material. Ethylene may be used in substantially pure form or admixed with one or more of nitrogen, methane, ethane, carbon dioxide and water in the form of steam or one or more of hydrogen, C3/C4 alkenes or alkanes. The ethylene in the combined feed to the reactor may be at least 60 mole %.
The oxygen-containing gas may suitably be air or a gas richer or poorer in molecular oxygen than air. Suitably, the gas may be oxygen diluted with a suitable diluent, for example, nitrogen, argon or carbon dioxide. Preferably the gas is oxygen.
The acetic acid is introduced into the reactor in liquid form. Optionally, a portion of the acid may be introduced in the vapour form. The acetic acid is preferably crude acetic acid. Suitably, the liquid acetic acid may be introduced into the fluid bed reactor by any suitable injection means, for example a nozzle which may be a gas-induced atomising nozzle or liquid-only spray-type nozzles. One or more nozzles may be used for this purpose. Additionally, recycled acetic acid may be introduced into the reactor. The recycled acetic acid may be pre-mixed with the crude acetic acid or may be introduced into the reactor using a separate injection means. The recycled acetic acid may suitably comprise water. Suitably, the water concentration in the recycle stream is such that the water concentration being fed into the reactor is less than 6% by weight, preferably less than 4% more preferably, less than 3% of the total acid and water stream fed into the reactor. The usual precautions for introducing a liquid into a fluid bed should be taken, such as avoiding cold surfaces.
It is generally known in such catalytic processes that catalyst activity will decrease with time for various reasons. In particular, due to the volatile nature of the co-promoter, the level of co-promoter in the catalyst material decreases with time. This results in loss of catalyst activity and a loss in selectivity. In order to maintain a constant concentration of co-promoter on the catalyst, the concentration being less than 6 wt %, fresh co-promoter may be added to the catalyst during the reaction. This may suitably be carried out by adding the co-promoter material to the liquid acetic acid feed or to the liquid recycle acetic acid. Alternatively, the additional co-promoter may be introduced as a solution e.g. in water or in acid directly by spraying into the reactor through a suitable injection means such as a nozzle. In each case, the co-promoter contacts the catalyst material.
The process is carried out in a fluid bed reactor and may suitably be operated at a temperature from 100 to 400xc2x0 C., preferably 140 to 210xc2x0 C. and a pressure of 105 to 2xc3x97106 Pa gauge (1 to 20 barg), preferably 6xc3x97105 to 1.5xc3x97106 Pa gauge (6 to 15 barg), especially 7xc3x97105 to 1.2xc3x97106 Pa gauge (7 to 12 barg).